The present invention relates to a method and apparatus for processing a substrate and detecting an endpoint of a process performed on a substrate.
In integrated circuit fabrication, semiconductor, dielectric, and conductor materials, such as for example, polysilicon, silicon dioxide, and aluminum layers are deposited on a substrate and etched to form patterns of gates, vias, contact holes, or interconnect lines. The layers are typically deposited by chemical vapor deposition, physical vapor deposition, or thermal oxidation processes. In the etching process, a patterned mask layer of photoresist or hard mask is formed on the deposited layer by photolithographic methods, and the exposed portions of the layer are etched by energized halogen gases, such as Cl.sub.2, HBr, and BCl.sub.3, and which also often include passivating gases, such as N.sub.2, which are used to generate passivating deposits on the sidewalls of freshly etched features to provide anisotropic etching.
The process chambers used in the deposition and the etching processes are periodically cleaned to remove residue deposits and contaminants that are formed on the walls, components, and internal surfaces of the chamber, otherwise these deposits flake off and contaminate the substrate. In etching processes, after etching every 100 to 300 wafers, the chamber is opened to the atmosphere and cleaned in a "wet-cleaning" process, in which an operator uses an acid or solvent to scrub off or dissolve accumulated etch residue on the chamber walls. After cleaning, the chamber is pumped down in a vacuum for 2 to 3 hours to outgas volatile species, and a series of etching runs are performed on dummy wafers until the chamber provides consistent etching properties. In the competitive semiconductor industry, the downtime of the etching chamber during the cleaning process can substantially increase the cost/substrate and is highly undesirable. Also, because the wet cleaning process is manually performed, the cleanliness of the chamber surfaces often vary from one cleaning session to another. Thus it is desirable to have a semiconductor process that reduces or eliminates the residue deposits that are formed on the surfaces inside the chamber.
Another problem with conventional etching processes arises because it is difficult to etch very thin layers on the substrate because the etching processes can uncontrollably etch through the thin layer and damage the underlying layer. Especially for silicon oxide (gate oxide) dielectric layers, it is desirable for the remaining thickness of the dielectric layer to be close to a nominal value and for the etching process not to damage the underlying polysilicon or silicon. The gate oxide layer is becoming thinner and thinner in high speed integrated circuits, making it more difficult to accurately etch through the overlying polysilicon layer without overetching into the gate oxide layer, particularly when halogen and fluorine containing gases (that etch through polysilicon with high etch rates) are used. It is desirable to stop the etching process on the gate oxide layer because polysilicon can undergo charge damage and lattice structural damage upon exposure to the energetic plasma ions.
Endpoint detection methods are used to measure the endpoint of the etching process to prevent etching through the overlayers by stopping the etching process before the overlayers are etched through. Endpoint measurement techniques include for example, plasma emission analysis in which an emission spectra of a plasma in the chamber is analyzed to determine a change in chemical composition that corresponds to a change in the chemical composition of the layer being etched, as taught in U.S. Pat. No. 4,328,068 which is incorporated herein by reference. However, plasma emission methods detect etching endpoint only after an overlayer having a particular chemical composition is etched through because they rely on the change in chemical compositions in the underlayer to obtain a change in emission spectra. Furthermore, residue deposits that are formed on the emission monitoring window tend to block or selectively filter the optical emission spectra, resulting in endpoint detection errors. In addition, the sensitivity of plasma emission methods are a function of the etch rate and the total area being etched and are difficult to detect for slow etching processes and smaller etch areas, especially for etching of small contact openings.
An endpoint detection system that measures a process endpoint before processing of entire layer is completed is ellipsometry. In this method, a polarized light beam is reflected off the surface of a layer being etched and is analyzed to determine a phase shift and a change in magnitude of the reflected light that occurs upon etching through the layer, as for example disclosed in U.S. Pat. Nos. 3,874,797 and 3,824,017, both of which are incorporated herein by reference. Polarized light filters are used to measure the change in phase of the polarized light beam that is reflected from the surface of the substrate. However, ellipsometry measurements are also more complicated because have need to measure both the magnitude (.DELTA.) and the phase (.psi.) of the reflected wavelength to monitor changes in the etching process. It is difficult to obtain accurate ellipsometry readings of a patterned wafer surface, as for example, explained in Multiwavelength Ellipsometry for Real-Time Process Control of the Plasma Etching of Patterned Samples, Maynard Layadi and Tseng-Chung Li, J. Vac. Sci. Technol. B. 15(1), January/February 1997, which concludes that only a light beam having multiple wavelengths will give accurate layer thickness measurements. In addition, residue deposits that are formed on the transparent window of the chamber change the polarization of the light beam passing through the window, giving rise to erroneous measurements in ellipsometric endpoint detection methods.
Yet another problem with prior art endpoint detection methods arises from the method in which the process endpoint is mathematically determine from a process signal. Typically, the time derivative of a measured light signal is compared with that of a reference signal, and a change in derivative of the two signals is computed to determine a measurably different signal condition relative to a prior signal condition. However, a finite time must pass before the change in the derivative of the two signals can be computed or any other different mathematical condition is recognized, causing a time delay in which the thin underlying layers can be etched through, especially for aggressive etchant gas chemistries. The time delay can also result in undesirable charging or lattice damage of the underlying layers, especially for underlying polysilicon layers. Also, if the selected variation in derivative signals is too small, the fabrication process may never be terminated, and if it is too large, the process may be prematurely terminated.
Thus it is desirable to have an endpoint detection method that terminates a semiconductor process as soon as the desired thickness of a layer being processed on the substrate is achieved, and without damaging the underlying layers. It is further desirable to have an endpoint detection system that provides a signal prior to etching through, or deposition of, an entire layer to allow the etching or deposition process to be changed before completion of the process. It is also desirable to have an endpoint measurement system which measures a change in thickness of a layer being processed, with high resolution, low signal to noise ratio, and high reliability, independent of the strength of the light signal from the substrate, or that is transmitted through the chamber windows. It is also desirable to have a semiconductor etching process that can rapidly etch through an overlayer, while generating little or no deposits on the chamber surfaces, even after sequentially processing multiple batches of wafers in the chamber.